Silicone rubber is used for various uses in a wide field, because of its excellent performance in heat-resistance, fire retardance, chemical stability, weatherability, radiation resistance, electrical characteristics, and so on. In particular, since silicone rubber is physiologically inactive and has little response to tissue when silicone rubber is touched with a living organism, silicone rubber has been utilized as a material for medical instruments such as various catheters for medical appliances.
A medical catheter is a tube which is inserted into a body cavity such as cavum thoracis or abdominal cavity, a lumen part such as alimentary canal or ureter, or a blood vessel, to be used for draining body fluid or injection infusion of drug solution, nutritional tonic, and contrast media. A medical catheter necessitates wound-resistance (tear-resistance), kink-resistance (tensile strength), transparency, and flexibility (stretching-extensibility), in addition to biocompatibility. As a use of medical catheter, specifically, for example, a drainage tube of an evacuator for draining and removing postoperative sanguis or pus, and a tube for postoperative nutrition such as percutaneous endoscopic gastrostomy (PEG) are exemplary examples. In addition, in order to produce silicone rubber in an extra-fine tube shape for catheter, silicone rubber composition as a silicone rubber material should have extrusion moldability.
As materials for a medical catheter, soft polyvinyl chloride has been generally used in addition to silicone rubber. Although silicone rubber is superior to polyvinyl chloride and so on, in biocompatibility and flexibility, it is required to improve the strength of silicone rubber such as tear strength or tensile strength, and in particular, tear strength.
If tear strength is not sufficient, then the catheter may be broken by scar caused by a needle or cutting tool during carrying out a surgical operation, or, if tensile strength is not sufficient, then the catheter may be bent to be yielded and kinked, and thereby distribution in the catheter of body fluid to be drained or drug solution to be infused may be stagnated.
Thus various methods have been proposed in order to improve the tear strength and the tensile strength of silicone rubber (for example, Patent Documents 1-7). As a specific method to provide high tear characteristics to silicone rubber, adding inorganic filler such as silica fine particles, or uneven distribution of crosslinking density (in the silicone rubber system, both the area of high-crosslinking density and the area of low-crosslinking are distributed) are specific methods. It is thought that the improvement of tear characteristics by uneven distribution of crosslinking density is because the area of high-crosslinking density serves as resistance against the tear internal force.
For example, Patent Document 1 discloses curable silicone rubber composition including high viscosity organopolysiloxane (raw rubber (A)) having a low vinyl group as a main component, low viscosity organopolysiloxane (silicone oil (B)) having a high vinyl group content, vinyl group containing organopolysiloxane copolymer (vinyl group containing silicone resin (C)), organohydrogensiloxane (cross-linking agent (D)), platinum or a platinum compound (hardening catalyst (E)) and fine-powdery silica (filler (F)).
In addition, there is an example which embodied a small-angle X-ray scattering measurement of acrylic fiber as an example which analyzed a polymer material by an X-ray radio scattering measurement (Patent Document 8); however, conformation analysis by an X-ray radio scattering measurement has not still conducted structural analysis of silicone rubber composition.